JP3681893B2 - Wavelength conversion element and method for manufacturing wavelength conversion element - Google Patents

Wavelength conversion element and method for manufacturing wavelength conversion element Download PDF

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JP3681893B2
JP3681893B2 JP09239898A JP9239898A JP3681893B2 JP 3681893 B2 JP3681893 B2 JP 3681893B2 JP 09239898 A JP09239898 A JP 09239898A JP 9239898 A JP9239898 A JP 9239898A JP 3681893 B2 JP3681893 B2 JP 3681893B2
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film
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knbo
wavelength conversion
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JPH11271826A (en
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篤 尾上
綾子 吉田
清文 竹間
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Pioneer Corp
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/355—Non-linear optics characterised by the materials used
    • G02F1/3551—Crystals
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/37—Non-linear optics for second-harmonic generation
    • G02F1/377—Non-linear optics for second-harmonic generation in an optical waveguide structure
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/353—Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
    • G02F1/3544—Particular phase matching techniques
    • G02F1/3546—Active phase matching, e.g. by electro- or thermo-optic tuning
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/365—Non-linear optics in an optical waveguide structure

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  • Nonlinear Science (AREA)
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  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Description

【０００１】
【発明の属する技術分野】
本発明は、レーザ光等の光ビームの波長を変換し、例えば、第２次高調波等を得るための波長変換素子及びその製造方法の技術分野に属し、より詳細には、MgＯよりなる基板を有し、ＫNbＯ3膜を光導波路として構成されている波長変換素子及びその製造方法の技術分野に属する。
【０００２】
【従来の技術】
従来、レーザ光等を波長変換してより短波長のレーザ光等を得るために、入射したレーザ光等の第２次高調波を出射する波長変換素子がある。
【０００３】
そして、当該波長変換素子として一般的に知られているものに、MgＯよりなる基板上にＫNbＯ3膜を光導波路として結晶成長させ、当該成長させたＫNbＯ3膜の表面に薄膜電極と光閉じ込め用の例えばストライプ状のTiＯ2膜とを形成した構成のものがある。
【０００４】
この構成を有する波長変換素子を用いれば、ＫNbＯ3膜が、非線形光学定数が大きく且つ光損傷耐性も高いという特性を有することから、高効率で且つ寿命の長い波長変換素子を得ることができる。
【０００５】
【発明が解決しようとする課題】
しかしながら、上記の構成を有する従来の波長変換素子においては、光導波路材料としてのＫNbＯ3の格子定数と基板材料としてのMgＯの格子定数との整合が取れないために、基板上に成長させたＫNbＯ3膜内に内部歪みが生じ、光伝播時に損失が生じるという問題点があった。
【０００６】
また、MgＯ基板上にＫNbＯ3膜を成長させた時に、本来必要な結晶方位を有する結晶面（（００１）面）の他に不必要な方位の結晶面（例えば、（０１１）面）が同時に成長してしまい、この場合には波長変換の変換効率が低下してしまうという問題点があった。
【０００７】
そこで、本発明は、上記各問題点に鑑みて為されたもので、その課題は、MgＯ基板と、光導波路としてのＫNbＯ3膜とを有する波長変換素子において、内部歪みが少なく、且つ波長変換の変換特性が良好な波長変換素子及び当該波長変換素子の製造方法を提供することにある。
【０００８】
【課題を解決するための手段】
上記の課題を解決するために、請求項１に記載の発明は、MgＯよりなる基板と、光が伝播するＫNbＯ3膜と、前記基板と前記ＫNbＯ3膜との間に形成されたＫTaXNb(1-X)Ｏ3膜と、を備え、前記ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面で４０原子％以上６０原子％以下であると共に前記ＫNbＯ3膜に接する面で０％となり、且つ、前記ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面から前記ＫNbＯ3膜に接する面に向かう方向に減少している。
【０００９】
請求項１に記載の発明の作用によれば、MgＯよりなる基板と、光が伝播するＫNbＯ3膜との間にＫTaXNb(1-X)Ｏ3膜が形成されている。
【００１０】
このとき、ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の基板に接する面で４０原子％以上６０原子％以下であると共にＫNbＯ3膜に接する面で０原子％となり、且つ、当該ＫTaXNb(1-X)Ｏ3膜の基板に接する面からＫNbＯ3膜に接する面に向かう方向に減少している。
【００１１】
よって、ＫNbＯ3膜の結晶性が向上し、ＫNbＯ3膜内に良好な波長変換機能を有する結晶面がより多く含まれることとなると共に、ＫNbＯ3膜とＫTaXNb(1-X)Ｏ3膜との間における格子不整合及びＫTaXNb(1-X)Ｏ3膜と基板との間における格子不整合が夫々緩和される。
【００１２】
上記の課題を解決するために、請求項２に記載の発明は、請求項１に記載の波長変換素子において、前記ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面と前記ＫNbＯ3膜に接する面との間で連続的に変化している。
【００１３】
請求項２に記載の発明の作用によれば、請求項１に記載の発明の作用に加えて、ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の基板に接する面とＫNbＯ3膜に接する面との間で連続的に変化しているので、組成がＫTaXNb(1-X)Ｏ3膜からＫNbＯ3膜に連続的に移行することとなり、ＫNbＯ3膜における結晶性がより向上すると共に格子不整合がより緩和される。
【００１４】
上記の課題を解決するために、請求項３に記載の発明は、請求項１又は２に記載の波長変換素子において、前記ＫNbＯ3膜が、ＫNbＯ3結晶における（００１）面を主面としている。
【００１５】
請求項３に記載の発明の作用によれば、請求項１又は２に記載の発明の作用に加えて、ＫNbＯ3膜が、ＫNbＯ3結晶における（００１）面を主面としているので、ＫNbＯ3結晶における大きい非線形光学定数を利用して波長変換することができる。
【００１６】
上記の課題を解決するために、請求項４に記載の発明は、請求項１から３のいずれか一項に記載の波長変換素子において、前記ＫTaXNb(1-X)Ｏ3膜の厚さが３００Å以上５００Å以下とされている。
【００１７】
請求項４に記載の発明の作用によれば、請求項１から３のいずれか一項に記載の発明の作用に加えて、ＫTaXNb(1-X)Ｏ3膜の厚さが３００Å以上５００Å以下であるので、ＫTaXNb(1-X)Ｏ3膜の厚さを必要最小限とすることができる。
【００１８】
上記の課題を解決するために、請求項５に記載の発明は、薄膜を積層することによりに形成される波長変換素子の製造方法において、MgＯよりなる基板上にＫTaXNb(1-X)Ｏ3膜を成長させる第１成長工程であって、成長された前記ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面で４０原子％以上６０原子％以下であると共に前記ＫNbＯ3膜に接する面で０原子％となり、且つ、前記ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面から前記ＫNbＯ3膜に接する面に向かう方向に減少するように当該ＫTaXNb(1-X)Ｏ3膜を成長させる第１成長工程と、前記成長したＫTaXNb(1-X)Ｏ3膜の表面に、光が伝播するＫNbＯ3膜を更に成長させる第２成長工程と、を備える。
【００１９】
請求項５に記載の発明の作用によれば、MgＯよりなる基板上にＫTaXNb(1-X)Ｏ3膜を成長させる第１成長工程において、成長されたＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の基板に接する面で４０原子％以上６０原子％以下であると共にＫNbＯ3膜に接する面で０原子％となり、且つ、当該ＫTaXNb(1-X)Ｏ3膜の基板に接する面からＫNbＯ3膜に接する面に向かう方向に減少するように当該ＫTaXNb(1-X)Ｏ3膜を成長させる。
【００２０】
次に、第２成長工程において、成長したＫTaXNb(1-X)Ｏ3膜の表面に、光が伝播するＫNbＯ3膜を更に成長させる。
【００２１】
よって、ＫNbＯ3膜と基板の間にＫTaXNb(1-X)Ｏ3膜が形成されるので、ＫNbＯ3膜の結晶性が向上し、当該ＫNbＯ3膜内に良好な波長変換機能を有する結晶面がより多く含まれると共に、ＫNbＯ3膜とＫTaXNb(1-X)Ｏ3膜との間における格子不整合及びＫTaXNb(1-X)Ｏ3膜と基板との間における格子不整合が夫々緩和された波長変換素子を製造することができる。
【００２２】
上記の課題を解決するために、請求項６に記載の発明は、請求項５に記載の製造方法において、前記第１成長工程において、前記ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面と前記ＫNbＯ3膜に接する面との間で連続的に変化するように当該ＫTaXNb(1-X)Ｏ3膜を成長させる。
【００２３】
請求項６に記載の発明の作用によれば、請求項５に記載の発明の作用に加えて、第１成長工程において、ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の基板に接する面とＫNbＯ3膜に接する面との間で連続的に変化するように当該ＫTaXNb(1-X)Ｏ3膜を成長させるので、組成がＫTaXNb(1-X)Ｏ3膜からＫNbＯ3膜に連続的に移行することとなり、ＫNbＯ3膜における結晶性がより向上すると共に格子不整合がより緩和される。
【００２４】
上記の課題を解決するために、請求項７に記載の発明は、請求項５又は６に記載の製造方法において、前記第２成長工程において、前記ＫNbＯ3膜を、ＫNbＯ3結晶における（００１）面を主面として結晶成長させる。
【００２５】
請求項７に記載の発明の作用によれば、請求項５又は６に記載の発明の作用に加えて、第２成長工程において、ＫNbＯ3膜を、ＫNbＯ3結晶における（００１）面を主面として結晶成長させるので、ＫNbＯ3結晶における大きい非線形光学定数を利用して波長変換可能な波長変換素子を製造できる。
【００２６】
上記の課題を解決するために、請求項８に記載の発明は、請求項５から７のいずれか一項に記載の製造方法において、前記第１成長工程において、前記ＫTaXNb(1-X)Ｏ3膜を厚さが３００Å以上５００Å以下となるように結晶成長させる。
【００２７】
請求項８に記載の発明の作用によれば、請求項５から７のいずれか一項に記載の発明の作用に加えて、第１成長工程において、ＫTaXNb(1-X)Ｏ3膜を厚さが３００Å以上５００Å以下となるように結晶成長させるので、ＫTaXNb(1-X)Ｏ3膜の厚さを必要最小限としてＫNbＯ3膜を結晶成長させることができる。
【００２８】
【発明の実施の形態】
次に、本発明に好適な実施の形態について、図面に基づいて説明する。なお、以下に説明する実施形態は、入射されるレーザ光を波長変換して第２次高調波を得る、いわゆるチャネル型の波長変換素子に対して本発明を適用した場合の実施形態である。
【００２９】
始めに、本発明の波長変換素子において、光学的非線形性を用いて波長変換を行うＫNbＯ3膜（以下、単にＫＮ膜と称する。）について、図１を用いて説明する。
【００３０】
ＫN膜は、上述のように光損傷に対する耐性が高い強誘電体であるが、その結晶構造は、二辺の長さが異なる直方体格子の斜方晶系結晶としての構造を有している。そして、その格子定数は、図１に示すようにａが３．９６９２Å、ｂが５．６８９６Å、ｃが５．７２５６Åである。
【００３１】
また、斜方晶系の分極と非線形感受テンソル及び電界との関係については、下記式１に示される関係となっている。ここで、式（１）においてＰは分極成分であり、ｄは２次非線形光学定数であり、Ｅは基本波電界成分であり、各添え字のａ、ｂ及びｃは軸方向を示す。
【００３２】
【数１】
従って、当該ＫN膜をエピタキシャル成長させ、これを波長変換素子の導波路として利用する場合には、当該ＫN膜の（００１）面すなわち（ａ−ｂ）面で成長させ、これにより、各非線形光学定数のうち大きな値を有する非線形光学定数ｄ32を利用することとなる。そして、この場合、図１に示すように、ａ軸方向へ伝播しａ−ｂ面内で振動する基本波から、ａ軸方向へ伝播しａ−ｃ面内で振動する第２次高調波が得られる。
【００３３】
ここで、ＫN膜の各非線形光学定数を表１に示す。
【００３４】
【表１】

Figure 0003681893
ｄ33＝−２７．４±０．３
ｄ32＝−１８．３±０．３
ｄ32＝−２０．５±０．３（λ＝８６０ｎｍ）
ｄ31＝−１５．８±０．３
ｄ24＝１７．１±０．４
ｄ16＝１６．５±０．４
ｄ15＝１６．５±０．４
（単位；ｐｍ／Ｖ）（基本波波長λ＝１．０６μｍ）
【００３５】
次に、実施形態の波長変換素子に用いられている導波路としての上記ＫＮ膜及び基板を含む部分の構成について、図２を用いて説明する。
【００３６】
実施形態の波長変換素子においては、図２（ａ）に示すように、基板１上に厚さ５００ÅのＫTaXNb(1-X)Ｏ3膜（以下、単にＫＴＮ膜と称する）２を挟んで、導波路としての厚さ約９０００ÅのＫＮ膜３が形成されている。ここで、当該ＫＴＮ膜２及びＫＮ膜３は、後述する条件の下、有機金属気相成長法（以下、ＭＯＣＶＤ（Metal Organic Chemical Vapor Deposition）法と称する。）により基板１上に夫々結晶成長させられたものである。また、実施形態の基板１は、MgＯ（１１０）よりなる基板を用いている。
【００３７】
更に、実施形態のＫＴＮ膜２においては、図２（ｂ）に示すように、当該ＫＴＮ膜２におけるタンタルTaの混合比Ｘの値が、当該ＫＴＮ膜２の基板１に接する面で５０原子％であると共にＫN膜３に接する面で０原子％となり、且つ、基板１に接する面からＫN膜３に接する面に向かう方向に単調減少で変化するように形成されている。
【００３８】
ここで、図３に示すＫＮ膜３の（００１）面の格子定数（図３（ａ）参照）と基板１（MgＯ）の（１１０）面の格子定数（図３（ｂ）参照）とは相互に異なるわけであるが、上記ＫＴＮ膜２が基板１とＫＮ膜３との間に介在しており、且つ、そのタンタルTaの含有率が基板１側の面で５０原子％でありそこから漸減してＫＮ膜３側の面で０原子％であることから、上記基板１とＫＮ膜３との間の格子定数の齟齬がＫＮ膜３の結晶性に与える影響が緩和され、よって、ＫＮ膜３を基板１上に直接成長させた場合に格子定数の差に起因して生じるＫＮ膜３内の内部歪みが極めて小さくなっている。
【００３９】
次に、本実施形態のＫＴＮ膜２を挟んでＫＮ膜３を成長させた場合の、当該ＫＮ膜３における配向性について、図４を用いて説明する。なお、図４は、ＭgＯ基板上に、タンタルTaの含有率を種々に変化させたＫＴＮ膜を夫々別個に気相成長させた場合の、当該成長させた夫々のＫＴＮ膜のＸ線回折分析結果を示すものである。
【００４０】
図４から明らかなように、タンタルTaを全く含まない膜をＭgＯ基板上に成長させた場合（すなわち、上記ＫＮ膜を直接ＭgＯ基板上に成長させた場合）には、波長変換に用いられるべき（００１）面（図４中符号ａ参照）だけでなく、（０１１）面や（０２２）面をも多く含んで成長してしまう。
【００４１】
これに対して、例えば、タンタルTaを５０原子％含有させて成長させたＫＴＮ膜では、図４中符号ｂで示すように、当該ＫＴＮ膜中に（００１）面が顕著に多く成長する。このことは、すなわち、当該タンタルTaを５０原子％含有させて成長させたＫＴＮ膜上に重ねてＫＮ膜を気相成長させた場合に、当該成長させたＫＮ膜内にも（００１）面が顕著に成長することを示している。
【００４２】
ここで、本実施形態の波長変換素子では、ＫＴＮ膜２におけるタンタルTaの含有率を、基板１側の面で５０原子％とすると共にＫＮ膜３側の面で０原子％とし、且つＫＮ膜３側に向かって漸次減少させているので、結果としてＫＴＮ膜２とＫＮ膜３とがその組成上連続的に積層されることとなる。よって、このことと上述したタンタルTaの含有率が５０原子％のときに（００１）面が顕著に成長することとが合間って、本実施形態のＫＴＮ膜２上に成長させられたＫＮ膜３では、当該（００１）面が更に顕著に多く含まれている。
【００４３】
なお、ＫＴＮ膜２の基板１側の面におけるタンタルTaの含有率（すなわち、タンタルTaの含有率の最大値）については、図４から明らかなように、上述した５０原子％の場合を含めて４０原子％乃至６０原子％の範囲であれば、いずれの場合でも、その上に積層されたＫＮ膜３では（００１）面が顕著に多く含まれることとなる。
【００４４】
次に、実施形態の波長変換素子を製造するための製造工程について、図５を用いて説明する。なお、図５は、当該波長変換素子が製造される過程を順を追って示す断面図である。
【００４５】
図５に示すように、実施形態の波長変換素子を製造する際には、始めに、予め製造されているＭgＯ（１１０）面よりなる基板１（図５（ａ）参照）上にＭＯＣＶＤ法によりＫＴＮ膜３（５００Å）とＫＮ膜３（約９０００Å）とを気相成長させる（図５（ｂ）参照）。
【００４６】
このときの結晶成長条件としては、例えば、ＫＴＮ膜２については、酸化物ＣＶＤ装置を用い、温度８５０℃、圧力５Torr（リアクター圧力）とし、材料ガスとしてジピバロイルメタナトカリウム（Ｋ（Ｃ11Ｈ19Ｏ2）。以下、Ｋ（ＤＰＭ）と称する。）、ペンタエトキシニオブ（Ｎb（ＯＣ2Ｈ5）5）及びペンタエトキシタンタル（Ta（ＯＣ2Ｈ5）5）を用いる。
【００４７】
一方、ＫＮ膜３については、同様に、酸化物ＣＶＤ装置を用い、温度８５０℃、圧力５Torr（リアクター圧力）とし、材料ガスとしてＫ（ＤＰＭ）、ペンタエトキシニオブ（Ｎb（ＯＣ2Ｈ5）5）を用いる。
【００４８】
より具体的に成膜工程を説明すると、ＭgＯ（１１０）を主面とする基枚１を酸化物ＣＶＤ装置の反応室に装填し、これを上記設定温度まで昇温し、次に反応室内部を上記設定気圧まで減圧し、出発材料としての上記各材料を酸化ＣＶＤ装置の気化器の夫々に装填する。次に、これら出発原料をそれぞれ上記設定温度に保つことにより昇華又は気化させ有機金属化合物ガスとし、これを夫々流量制御されたＡrキャリアガス及び酸化ガスＯ2を用いて加熱された基板１が配置された反応室へ層流として導き、基板１上にエピタキシャル層を祈出させる。
【００４９】
このとき、ＫＴＮ膜２の成膜においては、ペンタエトキシタンタル（Ta（ＯＣ2Ｈ5）5）の流量を当該ＫＴＮ膜２の成膜当初から漸次減少させ、当該成膜終了時には零として当該ＫＴＮ膜２におけるタンタルTaの含有量を、基板１側からＫＮ膜３側に向かって連続的に単調減少させる。
【００５０】
なお、この時に、図２（ｂ）に示すグラフに示したような組成のＫＴＮ膜２を得るべくタンタルTaの含有量を直線的に減少させる他に、例えば、曲線的な減少カーブを描くように減少させてもよい。このようにすれば、ＫＴＮ膜２におけるタンタルTaの組成の変化も曲線的な変化を示すこととなる。
【００５１】
ここで、出発材料からの各酸化物の生成には酸化反応をともなうため、反応ガスに一定量の酸素を添加してもよい。
【００５２】
上記ＫＴＮ膜２及びＫＮ膜３が形成されると、次に、その上に、スパッタ法又は真空蒸着法によりTiＯ2膜４を形成する（図５（ｃ）参照）。
【００５３】
ここで、当該TiＯ2膜４は、３次元導波路を形成するために空気よりも高屈折率の誘電体膜として形成されるものである。従って、TiＯ2膜４の膜厚としては、当該TiＯ2膜４自体がＫＮ膜３よりも高屈折率であるためその膜厚が光導波可能な膜厚となったのではＫＮ膜３内にレーザ光を閉じ込めることができないことから、カットオフ条件を満たすべく、例えば、約８００Å（ＫＮ膜３をシングルモード導波路とする場合）とされる。
【００５４】
なお、レーザ光閉じ込め用としては、上記TiＯ2膜４の他にＳiＯ2膜を用いることも可能であるが、この場合には、上述の理由からその膜厚は約２乃至３μｍ必要となる。
【００５５】
TiＯ2膜４が形成されると、次に、フォトリソグラフィー技術を用いて、当該形成されたTiＯ2膜４をレーザ光が伝播する方向に長いストライプ状にパターンニングし、閉じ込め層５を形成する（図５（ｄ）参照）。この時のエッチングには、例えば、ＲＩＥ（Reactive Ion Etching）法が用いられる。
【００５６】
そして、その後、TiＯ2膜４がエッチングされて除去された領域に、閉じ込め層５と平行に、レーザ光の基本波と第２次高調波との位相整合を取るためのストライプ状の電極６及び７を形成して（図５（ｅ）参照）波長変換素子Ｓが完成する。
【００５７】
なお、電極６及び７の材料としては、例えば、アルミニウム又はアルミニウム合金或いは金又は白金等を用いることができるが、波長変換素子としての動作時に高電界が印加されるため、実際には金又は白金の方が望ましい。
【００５８】
更に、上記各工程に加えて、閉じ込め層５の表面の一部に、入射光制御用の回折格子（図７符号８参照）を形成してもよい。
【００５９】
ここで、上述のようにして成膜したＫＮ膜３の表面の状態について、図６に示す図面代用写真を用いて説明する。なお、図６（ａ）はＫＴＮ膜２を用いずに基板１上に直接ＫＮ膜３を成膜した場合の当該ＫＮ膜３の表面状態を示す写真であり、図６（ｂ）はＫＴＮ膜２を挟んで基板１上にＫＮ膜３を成膜した場合の当該ＫＮ膜３の表面状態を示す写真である。
【００６０】
図６から明らかなように、ＫＴＮ膜２を介してＫＮ膜３を成膜した方が、ＫＴＮ膜２を用いないで成膜した場合に比して表面状態が木目細か且つ平滑になっている。このことと図４に示したＸ線回折結果とを考慮すると、ＫＴＮ膜２を間に挟んでＫＮ膜３を成膜した方が、所望の（００１）面を主面とするＫＮ膜３が成長し易いことがわかる。
【００６１】
次に、完成した波長変換素子Ｓを実際に動作させる場合の構成について、図７を用いて説明する。
【００６２】
図７に示すように、波長変換素子Ｓを動作させる場合には、上述した電極６及び７に対して電源９により電圧を印加して各電極間に電界を発生させ、当該電界によりＫＮ膜３の１次電気光学効果を用いて基本波モードの実行屈折率と第２次高調波モードの実行屈折率との位相整合を取って第２次高調波を発生させる。このとき、基本波は閉じ込め層５の短辺の一端側から入射し、第２次高調波及び基本波が当該短辺の他端側から出射することとなる。
【００６３】
【実施例】
次に、具体的な波長変換実験について説明する。
【００６４】
上述の製造構成により製造された波長変換素子Ｓ（ＫＮ層３の膜厚９０００Å、ＫＴＮ層２の膜厚５００Å、閉じ込め層５の厚さ８００Åで閉じ込め層５の基本波入射側の表面に回折格子８を有している。）を用いて、基本波（λ＝８６０ｎｍ）をＫＮ膜３にその断面から導波させることにより、良好な光閉じ込めが達成され、第２次高調波（λ＝４３０ｎｍ）が高変換効率で得られた。
【００６５】
更に、ＫＮ膜３の光損傷耐性の高さに起因して、高い出カまで安定した勤作が得られた。
【００６６】
以上説明したように、実施形態の波長変換素子Ｓによれば、ＫＴN膜２におけるタンタルTaの混合比Ｘの値が、当該基板１に接する面で５０原子％であると共にＫＮ膜３に接する面で０％となり、且つ、当該基板１に接する面からＫN膜３に接する面に向かう方向に単調減少しているので、ＫN膜３の結晶性が向上し、更にＫN膜３内に良好な波長変換機能を有する結晶面（（００１）面）がより多く含まれることとなると共に、ＫN膜３とＫＴＮ膜２との間における格子不整合及びＫＴN膜２と基板１との間における格子不整合が夫々緩和される。
【００６７】
よって、高効率で波長変換が可能になる。
【００６８】
また、ＫＴN膜２におけるタンタルTaの混合比Ｘの値が、当該ＫＴN膜２の基板１に接する面とＫN膜３に接する面との間で連続的に変化しているので、組成がＫＴN膜２からＫN膜３に連続的に移行することとなり、ＫN膜３における結晶性がより向上すると共に格子不整合がより緩和される。
【００６９】
更に、ＫN膜３が、ＫNbＯ3結晶における（００１）面を主面としているので、ＫNbＯ3結晶における大きい非線形光学定数を利用して波長変換することができる。
【００７０】
また、ＫＴN膜２の厚さが５００Åであるので、ＫＴN膜２の厚さを必要最小限とすることができる。
【００７１】
なお、上述の実施形態では、ＫＮ膜３と基板１との間にＫＴＮ膜２を挟む構成としたが、これ以外に、ＫＮ膜３と基板１との間の格子不整合を更に有効に解消するために、ＫＴＮ膜２に代えて、ＫTaNbRbＯ3膜を用いてもよい。この場合には、ルビジウムRbの格子定数が、ＫＮ膜３の格子定数と基板１の格子定数の間の値を有しているので、より有効に格子不整合を低減することができる。
【００７２】
また、同様に、ＫＴＮ膜２に代えて、ＫTaNbCsＯ3膜を用いてもよい。
【００７３】
また、ＫＴＮ膜２の膜厚については、上述の実施形態では５００Åとしたが、これ以外に、ＫＴＮ膜２及びＫＮ膜３の結晶の配向性を向上させることができる膜厚であれば、ＫＴＮ膜２はなるべく薄くすることが望ましく、この意味で、当該ＫＴＮ膜２の膜厚は３００Å以上５００Å以下であることが望ましい。
【００７４】
【発明の効果】
以上説明したように、請求項１に記載の発明によれば、ＫNbＯ3膜の結晶性が向上し、ＫNbＯ3膜内に良好な波長変換機能を有する結晶面がより多く含まれると共に、ＫNbＯ3膜とＫTaXNb(1-X)Ｏ3膜との間における格子不整合及びＫTaXNb(1-X)Ｏ3膜と基板との間における格子不整合が夫々緩和されるので、ＫNbＯ3膜における内部歪みが減少して結晶性が向上し、高効率で波長変換することができる。
請求項２に記載の発明によれば、請求項１に記載の発明の効果に加えて、ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の基板に接する面とＫNbＯ3膜に接する面との間で連続的に変化しているので、組成がＫTaXNb(1-X)Ｏ3膜からＫNbＯ3膜に連続的に移行することとなり、ＫNbＯ3膜における結晶性がより向上すると共に格子不整合がより緩和される。
【００７５】
請求項３に記載の発明によれば、請求項１又は２に記載の発明の効果に加えて、ＫNbＯ3膜が、ＫNbＯ3結晶における（００１）面を主面としているので、ＫNbＯ3結晶における大きい非線形光学定数を利用してより高効率で波長変換することができる。
【００７６】
請求項４に記載の発明によれば、請求項１から３のいずれか一項に記載の発明の効果に加えて、ＫTaXNb(1-X)Ｏ3膜の厚さが３００Å以上５００Å以下であるので、ＫTaXNb(1-X)Ｏ3膜の厚さを必要最小限とすることができる。
【００７７】
請求項５に記載の発明によれば、ＫNbＯ3膜の結晶性が向上し、当該ＫNbＯ3膜内に良好な波長変換機能を有する結晶面がより多く含まれることとなると共に、ＫNbＯ3膜とＫTaXNb(1-X)Ｏ3膜との間における格子不整合及びＫTaXNb(1-X)Ｏ3膜と基板との間における格子不整合が夫々緩和されるので、ＫNbＯ3膜における内部歪みが減少して結晶性が向上し、高効率で波長変換可能な波長変換素子を製造することができる。
【００７８】
請求項６に記載の発明によれば、請求項５に記載の発明の効果に加えて、ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の基板に接する面とＫNbＯ3膜に接する面との間で連続的に変化するように当該ＫTaXNb(1-X)Ｏ3膜を成長させるので、組成がＫTaXNb(1-X)Ｏ3膜からＫNbＯ3膜に連続的に移行することとなり、ＫNbＯ3膜における結晶性がより向上すると共に格子不整合がより緩和される。
【００７９】
請求項７に記載の発明によれば、請求項５又は６に記載の発明の効果に加えて、ＫNbＯ3膜を、ＫNbＯ3結晶における（００１）面を主面として結晶成長させるので、ＫNbＯ3結晶における大きい非線形光学定数を利用してより高効率で波長変換可能な波長変換素子を製造できる。
【００８０】
請求項８に記載の発明によれば、請求項５から７のいずれか一項に記載の発明の効果に加えて、ＫTaXNb(1-X)Ｏ3膜を厚さが３００Å以上５００Å以下となるように結晶成長させるので、ＫTaXNb(1-X)Ｏ3膜の厚さを必要最小限としてＫNbＯ3膜を結晶成長させることができる。
【図面の簡単な説明】
【図１】ＫＮ膜における結晶構造と光ビームの入射方向との関係を示す図である。
【図２】ＫＮ膜とＫＴＮ膜の構成を示す図であり、（ａ）は基板を含んだ構造を示す断面図であり、（ｂ）はＫＴＮ膜におけるタンタルの含有率の厚さに対する変化を示す図である。
【図３】ＫＮ膜とＭgＯ基板の結晶構造を示す図であり、（ａ）はＫNbＯ3（００１）面の単位結晶の大きさを示す図であり、（ｂ）はＭgＯ（１１０）面の単位結晶の大きさを示す図である。
【図４】ＭgＯ基板上のＫＴＮ膜におけるタンタルの含有率と結晶性の関係を示す図である。
【図５】波長変換素子の製造工程を示す図であり、（ａ）は基板を示す断面図であり、（ｂ）は基板上に積層されたＫＮ膜及びＫＴＮ膜を示す断面図であり、（ｃ）はＫＮ膜上に成膜されたTiＯ2膜を示す断面図であり、（ｄ）は閉じ込め層を形成した後の波長変換素子を示す断面図であり、（ｅ）は電極を形成した後の波長変換素子を示す断面図である。
【図６】ＫＮ膜の表面を示す図面代用写真であり、（ａ）はＭｇＯ基板上に直接成膜したＫＮ膜の表面を示す図面代用写真であり、（ｂ）はＫＴＮ膜を挟んで成膜したＫＮ膜の表面を示す図面代用写真である。
【図７】波長変換素子における波長変換動作を示す斜視図である。
【符号の説明】
１…基板
２…ＫＴＮ膜
３…ＫＮ膜
４…ＴiＯ2膜
５…閉じ込め層
６、７…電極
８…回折格子
９…電源
Ｓ…波長変換素子[0001]
BACKGROUND OF THE INVENTION
The present invention belongs to the technical field of a wavelength conversion element for converting the wavelength of a light beam such as a laser beam and obtaining, for example, a second harmonic and the like, and more specifically, a substrate made of MgO. KNbO Three The present invention belongs to a technical field of a wavelength conversion element having a film as an optical waveguide and a manufacturing method thereof.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, there is a wavelength conversion element that emits a second harmonic such as an incident laser beam in order to obtain a shorter wavelength laser beam or the like by converting the wavelength of the laser beam or the like.
[0003]
In addition, what is generally known as the wavelength conversion element is formed on a substrate made of MgO with KNbO. Three Crystal growth using the film as an optical waveguide, and the grown KNbO Three A thin film electrode on the surface of the film and, for example, striped TiO for optical confinement 2 There is a configuration in which a film is formed.
[0004]
If a wavelength conversion element having this configuration is used, KNbO Three Since the film has the characteristics that the nonlinear optical constant is large and the optical damage resistance is high, a wavelength conversion element with high efficiency and a long lifetime can be obtained.
[0005]
[Problems to be solved by the invention]
However, in the conventional wavelength conversion element having the above configuration, KNbO as an optical waveguide material is used. Three Since the lattice constant of MgO and the lattice constant of MgO as the substrate material cannot be matched, KNbO grown on the substrate Three There is a problem in that internal distortion occurs in the film and loss occurs during light propagation.
[0006]
In addition, KNbO on the MgO substrate Three When a film is grown, a crystal plane having an unnecessary orientation (for example, (011) plane) is simultaneously grown in addition to a crystal plane having an originally necessary crystal orientation ((001) plane). However, there is a problem that the conversion efficiency of wavelength conversion is lowered.
[0007]
Therefore, the present invention has been made in view of the above-mentioned problems, and its problems are an MgO substrate and KNbO as an optical waveguide. Three It is an object of the present invention to provide a wavelength conversion element having a film and a method of manufacturing the wavelength conversion element with less internal distortion and good wavelength conversion conversion characteristics.
[0008]
[Means for Solving the Problems]
In order to solve the above-described problems, the invention described in claim 1 includes a substrate made of MgO, and KNbO in which light propagates. Three Film, substrate and KNbO Three KTa formed between films X Nb (1-X) O Three And the KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three 40 atomic% or more and 60 atomic% or less on the surface in contact with the substrate of the film and the KNbO Three 0% on the surface in contact with the film, and KTa X Nb (1-X) O Three From the surface of the film in contact with the substrate, the KNbO Three It decreases in the direction toward the surface in contact with the film.
[0009]
According to the operation of the first aspect of the present invention, the substrate made of MgO and the KNbO through which light propagates. Three KTa between the membrane X Nb (1-X) O Three A film is formed.
[0010]
At this time, KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three It is 40 atomic% or more and 60 atomic% or less on the surface in contact with the substrate of the film and KNbO Three 0 atomic% on the surface in contact with the film, and the KTa X Nb (1-X) O Three KNbO from the surface of the film in contact with the substrate Three It decreases in the direction toward the surface in contact with the film.
[0011]
Therefore, KNbO Three The crystallinity of the film is improved and KNbO Three The crystal face having a good wavelength conversion function is contained in the film more and KNbO Three Membrane and KTa X Nb (1-X) O Three Lattice mismatch between films and KTa X Nb (1-X) O Three The lattice mismatch between the film and the substrate is alleviated.
[0012]
In order to solve the above-mentioned problem, the invention according to claim 2 is the wavelength conversion element according to claim 1, wherein the KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three The surface of the film in contact with the substrate and the KNbO Three It continuously changes between the surface in contact with the film.
[0013]
According to the operation of the invention described in claim 2, in addition to the operation of the invention described in claim 1, KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three The surface of the film in contact with the substrate and KNbO Three Since the composition changes continuously with the surface in contact with the film, the composition is KTa X Nb (1-X) O Three From membrane to KNbO Three The film will move continuously to the KNbO Three The crystallinity in the film is further improved and the lattice mismatch is more relaxed.
[0014]
In order to solve the above problem, the invention according to claim 3 is the wavelength conversion element according to claim 1 or 2, wherein the KNbO is used. Three The membrane is KNbO Three The (001) plane in the crystal is the main surface.
[0015]
According to the operation of the invention described in claim 3, in addition to the operation of the invention described in claim 1 or 2, KNbO Three The membrane is KNbO Three Since the (001) plane in the crystal is the main surface, KNbO Three Wavelength conversion can be performed using a large nonlinear optical constant in the crystal.
[0016]
In order to solve the above problems, the invention according to claim 4 is the wavelength conversion element according to any one of claims 1 to 3, wherein the KTa X Nb (1-X) O Three The thickness of the film is not less than 300 mm and not more than 500 mm.
[0017]
According to the operation of the invention described in claim 4, in addition to the operation of the invention described in any one of claims 1 to 3, KTa X Nb (1-X) O Three Since the thickness of the film is not less than 300 mm and not more than 500 mm, KTa X Nb (1-X) O Three The film thickness can be minimized.
[0018]
In order to solve the above-mentioned problems, the invention according to claim 5 is a method of manufacturing a wavelength conversion element formed by laminating thin films on a substrate made of MgO on KTa. X Nb (1-X) O Three A first growth step for growing a film, the grown KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three 40 atomic% or more and 60 atomic% or less on the surface in contact with the substrate of the film and the KNbO Three 0 atomic% on the surface in contact with the film, and the KTa X Nb (1-X) O Three From the surface of the film in contact with the substrate, the KNbO Three The KTa so as to decrease in the direction toward the surface in contact with the film. X Nb (1-X) O Three A first growth step for growing a film, and the grown KTa X Nb (1-X) O Three KNbO where light propagates on the surface of the film Three A second growth step for further growing the film.
[0019]
According to the operation of the invention described in claim 5, KTa is formed on the substrate made of MgO. X Nb (1-X) O Three In the first growth step of growing the film, the grown KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three It is 40 atomic% or more and 60 atomic% or less on the surface in contact with the substrate of the film and KNbO Three 0 atomic% on the surface in contact with the film, and the KTa X Nb (1-X) O Three KNbO from the surface of the film in contact with the substrate Three The KTa so as to decrease in the direction toward the surface in contact with the film. X Nb (1-X) O Three Grow the film.
[0020]
Next, in the second growth step, the grown KTa X Nb (1-X) O Three KNbO where light propagates on the surface of the film Three The film is grown further.
[0021]
Therefore, KNbO Three KTa between the film and the substrate X Nb (1-X) O Three Since the film is formed, KNbO Three The crystallinity of the film is improved and the KNbO Three More crystal planes having a good wavelength conversion function are contained in the film, and KNbO Three Membrane and KTa X Nb (1-X) O Three Lattice mismatch between films and KTa X Nb (1-X) O Three A wavelength conversion element in which the lattice mismatch between the film and the substrate is alleviated can be manufactured.
[0022]
In order to solve the above-described problem, the invention according to claim 6 is the manufacturing method according to claim 5, wherein in the first growth step, the KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three The surface of the film in contact with the substrate and the KNbO Three The KTa is continuously changed between the surface in contact with the film. X Nb (1-X) O Three Grow the film.
[0023]
According to the operation of the invention described in claim 6, in addition to the operation of the invention described in claim 5, in the first growth step, KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three The surface of the film in contact with the substrate and KNbO Three The KTa is continuously changed between the surface in contact with the film. X Nb (1-X) O Three Since the film is grown, the composition is KTa X Nb (1-X) O Three From membrane to KNbO Three The film will move continuously to the KNbO Three The crystallinity in the film is further improved and the lattice mismatch is more relaxed.
[0024]
In order to solve the above-mentioned problem, the invention according to claim 7 is the manufacturing method according to claim 5 or 6, wherein in the second growth step, the KNbO is used. Three Membrane with KNbO Three Crystal growth is performed with the (001) plane of the crystal as the principal plane.
[0025]
According to the operation of the invention described in claim 7, in addition to the operation of the invention described in claim 5 or 6, in the second growth step, KNbO Three Membrane with KNbO Three Since the crystal is grown with the (001) plane in the crystal as the principal plane, KNbO Three A wavelength conversion element capable of wavelength conversion can be manufactured by utilizing a large nonlinear optical constant in the crystal.
[0026]
In order to solve the above problem, an invention according to an eighth aspect is the manufacturing method according to any one of the fifth to seventh aspects, wherein in the first growth step, the KTa X Nb (1-X) O Three The film is crystal-grown so that the thickness is 300 to 500 mm.
[0027]
According to the operation of the invention described in claim 8, in addition to the operation of the invention described in any one of claims 5 to 7, in the first growth step, KTa X Nb (1-X) O Three Since the film is grown to have a thickness of 300 to 500 mm, KTa X Nb (1-X) O Three KNbO with minimum film thickness Three The film can be crystal-grown.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Next, preferred embodiments of the present invention will be described with reference to the drawings. The embodiment described below is an embodiment when the present invention is applied to a so-called channel-type wavelength conversion element that obtains a second harmonic by converting the wavelength of incident laser light.
[0029]
First, in the wavelength conversion element of the present invention, KNbO that performs wavelength conversion using optical nonlinearity Three A film (hereinafter simply referred to as a KN film) will be described with reference to FIG.
[0030]
The KN film is a ferroelectric having high resistance to optical damage as described above, but its crystal structure is a structure of an orthorhombic crystal with a rectangular parallelepiped having different two side lengths. As shown in FIG. 1, the lattice constant is 3.9692 cm for a, 5.6896 cm for b, and 5.7256 cm for c.
[0031]
Further, the relationship among orthorhombic polarization, nonlinear susceptibility tensor, and electric field is represented by the following formula 1. Here, in Expression (1), P is a polarization component, d is a second-order nonlinear optical constant, E is a fundamental wave electric field component, and each subscript a, b, and c indicates an axial direction.
[0032]
[Expression 1]
Accordingly, when the KN film is epitaxially grown and used as a waveguide of the wavelength conversion element, it is grown on the (001) plane, that is, the (ab) plane of the KN film. Is a nonlinear optical constant d having a large value 32 Will be used. In this case, as shown in FIG. 1, from the fundamental wave that propagates in the a-axis direction and vibrates in the a-b plane, the second harmonic that propagates in the a-axis direction and vibrates in the a-c plane. can get.
[0033]
Here, each nonlinear optical constant of the KN film is shown in Table 1.
[0034]
[Table 1]
Figure 0003681893
d 33 = -27.4 ± 0.3
d 32 = -18.3 ± 0.3
d 32 = -20.5 ± 0.3 (λ = 860 nm)
d 31 = -15.8 ± 0.3
d twenty four = 17.1 ± 0.4
d 16 = 16.5 ± 0.4
d 15 = 16.5 ± 0.4
(Unit: pm / V) (fundamental wavelength λ = 1.06 μm)
[0035]
Next, the configuration of the portion including the KN film and the substrate as the waveguide used in the wavelength conversion element of the embodiment will be described with reference to FIG.
[0036]
In the wavelength conversion element of the embodiment, as shown in FIG. X Nb (1-X) O Three A KN film 3 having a thickness of about 9000 mm is formed as a waveguide with a film (hereinafter simply referred to as a KTN film) 2 interposed therebetween. Here, the KTN film 2 and the KN film 3 are crystal-grown on the substrate 1 by a metal organic chemical vapor deposition method (hereinafter referred to as MOCVD (Metal Organic Chemical Vapor Deposition) method) under the conditions described later. It is what was done. Moreover, the board | substrate 1 of embodiment uses the board | substrate which consists of MgO (110).
[0037]
Further, in the KTN film 2 of the embodiment, as shown in FIG. 2B, the value of the tantalum Ta mixing ratio X in the KTN film 2 is 50 atomic% on the surface of the KTN film 2 in contact with the substrate 1. In addition, it is 0 atomic% on the surface in contact with the KN film 3 and changes monotonously in the direction from the surface in contact with the substrate 1 toward the surface in contact with the KN film 3.
[0038]
Here, the lattice constant of the (001) plane of the KN film 3 shown in FIG. 3 (see FIG. 3A) and the lattice constant of the (110) plane of the substrate 1 (MgO) (see FIG. 3B) Although they are different from each other, the KTN film 2 is interposed between the substrate 1 and the KN film 3, and the tantalum Ta content is 50 atomic% on the surface on the substrate 1 side. Since it is gradually reduced to 0 atomic% on the surface on the KN film 3 side, the influence of the lattice constant wrinkle between the substrate 1 and the KN film 3 on the crystallinity of the KN film 3 is mitigated. When the film 3 is directly grown on the substrate 1, the internal strain in the KN film 3 caused by the difference in lattice constant is extremely small.
[0039]
Next, the orientation in the KN film 3 when the KN film 3 is grown with the KTN film 2 of the present embodiment interposed therebetween will be described with reference to FIG. FIG. 4 shows the results of X-ray diffraction analysis of each grown KTN film when KTN films having various tantalum Ta contents varied on the MgO substrate are separately vapor-phase grown. Is shown.
[0040]
As is apparent from FIG. 4, when a film containing no tantalum Ta is grown on the MgO substrate (that is, when the KN film is directly grown on the MgO substrate), it should be used for wavelength conversion. It grows including not only the (001) plane (see symbol a in FIG. 4) but also the (011) plane and (022) plane.
[0041]
On the other hand, for example, in a KTN film grown by containing 50 atomic% of tantalum Ta, (001) planes grow significantly more in the KTN film, as indicated by reference numeral b in FIG. That is, when a KN film is vapor-phase grown on a KTN film grown with 50 atomic% of tantalum Ta, the (001) plane is also present in the grown KN film. It shows a remarkable growth.
[0042]
Here, in the wavelength conversion element of this embodiment, the content of tantalum Ta in the KTN film 2 is 50 atomic% on the surface on the substrate 1 side and 0 atomic% on the surface on the KN film 3 side, and the KN film Since it is gradually decreased toward the third side, as a result, the KTN film 2 and the KN film 3 are continuously laminated due to their compositions. Therefore, the KN film grown on the KTN film 2 according to the present embodiment is combined with this fact that the (001) plane grows remarkably when the content of tantalum Ta is 50 atomic%. 3, the (001) plane is more significantly included.
[0043]
Note that the tantalum Ta content (that is, the maximum value of the tantalum Ta content) on the substrate 1 side surface of the KTN film 2 includes the above-described case of 50 atomic%, as is apparent from FIG. If it is in the range of 40 atomic% to 60 atomic%, in any case, the KN film 3 laminated thereon has a significantly large number of (001) planes.
[0044]
Next, a manufacturing process for manufacturing the wavelength conversion element of the embodiment will be described with reference to FIG. In addition, FIG. 5 is sectional drawing which shows the process in which the said wavelength conversion element is manufactured later on.
[0045]
As shown in FIG. 5, when the wavelength conversion element according to the embodiment is manufactured, first, a MOCVD method is performed on a substrate 1 (see FIG. 5A) made of a MgO (110) surface manufactured in advance. The KTN film 3 (500 mm) and the KN film 3 (about 9000 mm) are vapor-phase grown (see FIG. 5B).
[0046]
As the crystal growth conditions at this time, for example, for the KTN film 2, an oxide CVD apparatus is used, the temperature is 850 ° C., the pressure is 5 Torr (reactor pressure), and the material gas is dipivaloylmethanato potassium (K (C (C)). 11 H 19 O 2 ). Hereinafter referred to as K (DPM). ), Pentaethoxyniobium (Nb (OC 2 H Five ) Five ) And pentaethoxytantalum (Ta (OC 2 H Five ) Five ) Is used.
[0047]
On the other hand, for the KN film 3, similarly, an oxide CVD apparatus is used, the temperature is 850 ° C., the pressure is 5 Torr (reactor pressure), the material gas is K (DPM), pentaethoxyniobium (Nb (OC 2 H Five ) Five ) Is used.
[0048]
More specifically, the film forming process will be described. A substrate 1 having MgO (110) as a main surface is loaded into a reaction chamber of an oxide CVD apparatus, and the temperature is raised to the set temperature. Is reduced to the set atmospheric pressure, and the respective materials as starting materials are loaded into the vaporizers of the oxidation CVD apparatus. Next, each of these starting materials is sublimated or vaporized by maintaining the above set temperature to obtain an organometallic compound gas, which is controlled in flow rate by Ar carrier gas and oxidizing gas O, respectively. 2 As a laminar flow to the reaction chamber in which the heated substrate 1 is disposed, the epitaxial layer is prayed on the substrate 1.
[0049]
At this time, in the formation of the KTN film 2, pentaethoxytantalum (Ta (OC 2 H Five ) Five ) Is gradually reduced from the beginning of the formation of the KTN film 2 and is set to zero at the end of the film formation so that the tantalum Ta content in the KTN film 2 is continuously increased from the substrate 1 side toward the KN film 3 side. Monotonously decrease.
[0050]
At this time, in addition to linearly decreasing the content of tantalum Ta to obtain the KTN film 2 having the composition shown in the graph of FIG. 2B, for example, a curvilinear decrease curve is drawn. It may be decreased. In this way, the change in the composition of tantalum Ta in the KTN film 2 also shows a curvilinear change.
[0051]
Here, since the production of each oxide from the starting material involves an oxidation reaction, a certain amount of oxygen may be added to the reaction gas.
[0052]
When the above KTN film 2 and KN film 3 are formed, TiO 2 is then formed thereon by sputtering or vacuum evaporation. 2 A film 4 is formed (see FIG. 5C).
[0053]
Where the relevant TiO 2 The film 4 is formed as a dielectric film having a higher refractive index than air in order to form a three-dimensional waveguide. Therefore, TiO 2 As the film thickness of the film 4, the TiO 2 Since the film 4 itself has a higher refractive index than the KN film 3, the film thickness cannot be confined in the KN film 3 if the film thickness is such that the optical waveguide can be guided. Therefore, for example, the thickness is about 800 mm (when the KN film 3 is a single mode waveguide).
[0054]
In addition, the above-mentioned TiO is used for laser light confinement. 2 In addition to membrane 4, SiO 2 Although a film can be used, in this case, the film thickness is required to be about 2 to 3 μm for the reason described above.
[0055]
TiO 2 Once the film 4 is formed, the formed TiO is then used using a photolithography technique. 2 The film 4 is patterned in a stripe shape that is long in the direction in which the laser light propagates to form a confinement layer 5 (see FIG. 5D). For example, RIE (Reactive Ion Etching) is used for the etching at this time.
[0056]
And then TiO 2 Striped electrodes 6 and 7 for phase matching between the fundamental wave of the laser beam and the second harmonic are formed in parallel with the confinement layer 5 in the region where the film 4 has been etched away ( As shown in FIG. 5E, the wavelength conversion element S is completed.
[0057]
As materials for the electrodes 6 and 7, for example, aluminum, an aluminum alloy, gold, or platinum can be used. However, since a high electric field is applied during operation as a wavelength conversion element, gold or platinum is actually used. Is preferred.
[0058]
Further, in addition to the above steps, a diffraction grating for controlling incident light (see reference numeral 8 in FIG. 7) may be formed on a part of the surface of the confinement layer 5.
[0059]
Here, the state of the surface of the KN film 3 formed as described above will be described with reference to a drawing substitute photograph shown in FIG. 6A is a photograph showing the surface state of the KN film 3 when the KN film 3 is directly formed on the substrate 1 without using the KTN film 2, and FIG. 6B is a photograph showing the KTN film. 2 is a photograph showing the surface state of the KN film 3 when the KN film 3 is formed on the substrate 1 with 2 interposed therebetween.
[0060]
As is apparent from FIG. 6, the surface state is finer and smoother when the KN film 3 is formed via the KTN film 2 than when the KTN film 2 is not formed. . Considering this and the X-ray diffraction result shown in FIG. 4, the KN film 3 having the desired (001) plane as the principal surface is more formed when the KTN film 3 is formed with the KTN film 2 interposed therebetween. It turns out that it grows easily.
[0061]
Next, a configuration for actually operating the completed wavelength conversion element S will be described with reference to FIG.
[0062]
As shown in FIG. 7, when operating the wavelength conversion element S, a voltage is applied to the electrodes 6 and 7 by the power source 9 to generate an electric field between the electrodes, and the KN film 3 is generated by the electric field. Using the first-order electro-optic effect, phase matching is performed between the effective refractive index of the fundamental mode and the effective refractive index of the second harmonic mode to generate the second harmonic. At this time, the fundamental wave is incident from one end side of the short side of the confinement layer 5, and the second harmonic and the fundamental wave are emitted from the other end side of the short side.
[0063]
【Example】
Next, a specific wavelength conversion experiment will be described.
[0064]
The wavelength conversion element S manufactured according to the above-described manufacturing configuration (the thickness of the KN layer 3 is 9000 mm, the thickness of the KTN layer 2 is 500 mm, the thickness of the confinement layer 5 is 800 mm, and the diffraction grating is formed on the surface on the fundamental wave incident side of the confinement layer 5 Is used to guide the fundamental wave (λ = 860 nm) to the KN film 3 from its cross section, thereby achieving good optical confinement and the second harmonic (λ = 430 nm). ) Was obtained with high conversion efficiency.
[0065]
Furthermore, due to the high light damage resistance of the KN film 3, stable work was obtained up to high output.
[0066]
As described above, according to the wavelength conversion element S of the embodiment, the value of the mixing ratio X of tantalum Ta in the KTN film 2 is 50 atomic% on the surface in contact with the substrate 1 and the surface in contact with the KN film 3. 0%, and monotonically decreasing in the direction from the surface in contact with the substrate 1 to the surface in contact with the KN film 3, so that the crystallinity of the KN film 3 is improved, and a good wavelength is obtained in the KN film 3. More crystal planes having a conversion function ((001) plane) are included, and lattice mismatch between the KN film 3 and the KTN film 2 and lattice mismatch between the KTN film 2 and the substrate 1 are included. Are alleviated.
[0067]
Therefore, wavelength conversion can be performed with high efficiency.
[0068]
Further, since the value of the tantalum Ta mixing ratio X in the KTN film 2 continuously changes between the surface of the KTN film 2 in contact with the substrate 1 and the surface of the KTN film 2 in contact with the KN film 3, the composition of the KTN film 2 From 2 to the KN film 3, the crystallinity in the KN film 3 is further improved and the lattice mismatch is further relaxed.
[0069]
Furthermore, the KN film 3 is made of KNbO. Three Since the (001) plane in the crystal is the main surface, KNbO Three Wavelength conversion can be performed using a large nonlinear optical constant in the crystal.
[0070]
Further, since the thickness of the KTN film 2 is 500 mm, the thickness of the KTN film 2 can be minimized.
[0071]
In the above-described embodiment, the KTN film 2 is sandwiched between the KN film 3 and the substrate 1, but in addition to this, the lattice mismatch between the KN film 3 and the substrate 1 is more effectively eliminated. Therefore, instead of the KTN film 2, KTaNbRbO Three A membrane may be used. In this case, since the lattice constant of rubidium Rb has a value between the lattice constant of the KN film 3 and the lattice constant of the substrate 1, lattice mismatch can be reduced more effectively.
[0072]
Similarly, instead of the KTN film 2, KTaNbCsO Three A membrane may be used.
[0073]
Further, the thickness of the KTN film 2 is 500 mm in the above-described embodiment, but other than this, any film thickness that can improve the crystal orientation of the KTN film 2 and the KN film 3 is KTN. It is desirable to make the film 2 as thin as possible. In this sense, the thickness of the KTN film 2 is desirably 300 to 500 mm.
[0074]
【The invention's effect】
As described above, according to the first aspect of the present invention, KNbO Three The crystallinity of the film is improved and KNbO Three More crystal planes having a good wavelength conversion function are contained in the film, and KNbO Three Membrane and KTa X Nb (1-X) O Three Lattice mismatch between films and KTa X Nb (1-X) O Three Since the lattice mismatch between the film and the substrate is alleviated, KNbO Three The internal strain in the film is reduced, crystallinity is improved, and wavelength conversion can be performed with high efficiency.
According to the invention described in claim 2, in addition to the effect of the invention described in claim 1, KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three The surface of the film in contact with the substrate and KNbO Three Since the composition changes continuously with the surface in contact with the film, the composition is KTa X Nb (1-X) O Three From membrane to KNbO Three The film will move continuously to the KNbO Three The crystallinity in the film is further improved and the lattice mismatch is more relaxed.
[0075]
According to the invention of claim 3, in addition to the effect of the invention of claim 1 or 2, in addition to KNbO Three The membrane is KNbO Three Since the (001) plane in the crystal is the main surface, KNbO Three Wavelength conversion can be performed with higher efficiency by utilizing a large nonlinear optical constant in the crystal.
[0076]
According to the invention described in claim 4, in addition to the effect of the invention described in any one of claims 1 to 3, KTa X Nb (1-X) O Three Since the thickness of the film is not less than 300 mm and not more than 500 mm, KTa X Nb (1-X) O Three The film thickness can be minimized.
[0077]
According to the invention described in claim 5, KNbO Three The crystallinity of the film is improved and the KNbO Three The crystal face having a good wavelength conversion function is contained in the film more and KNbO Three Membrane and KTa X Nb (1-X) O Three Lattice mismatch between films and KTa X Nb (1-X) O Three Since the lattice mismatch between the film and the substrate is alleviated, KNbO Three A wavelength conversion element capable of wavelength conversion with high efficiency can be manufactured by reducing internal strain in the film and improving crystallinity.
[0078]
According to the invention described in claim 6, in addition to the effect of the invention described in claim 5, KTa X Nb (1-X) O Three The value of the Ta mixing ratio X in the film is the KTa X Nb (1-X) O Three The surface of the film in contact with the substrate and KNbO Three The KTa is continuously changed between the surface in contact with the film. X Nb (1-X) O Three Since the film is grown, the composition is KTa X Nb (1-X) O Three From membrane to KNbO Three The film will move continuously to the KNbO Three The crystallinity in the film is further improved and the lattice mismatch is more relaxed.
[0079]
According to the invention described in claim 7, in addition to the effect of the invention described in claim 5 or 6, in addition to KNbO Three Membrane with KNbO Three Since the crystal is grown with the (001) plane in the crystal as the principal plane, KNbO Three A wavelength conversion element capable of wavelength conversion with higher efficiency can be manufactured by using a large nonlinear optical constant in the crystal.
[0080]
According to the invention described in claim 8, in addition to the effect of the invention described in any one of claims 5 to 7, KTa X Nb (1-X) O Three Since the film is grown to have a thickness of 300 to 500 mm, KTa X Nb (1-X) O Three KNbO with minimum film thickness Three The film can be crystal-grown.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between a crystal structure in a KN film and an incident direction of a light beam.
FIGS. 2A and 2B are diagrams showing configurations of a KN film and a KTN film, FIG. 2A is a cross-sectional view showing a structure including a substrate, and FIG. 2B shows a change in the content of tantalum in the KTN film with respect to the thickness; FIG.
FIG. 3 is a diagram showing a crystal structure of a KN film and an MgO substrate, where (a) shows KNbO. Three It is a figure which shows the magnitude | size of the unit crystal of (001) plane, (b) is a figure which shows the magnitude | size of the unit crystal of MgO (110) plane.
FIG. 4 is a graph showing the relationship between tantalum content and crystallinity in a KTN film on an MgO substrate.
5A and 5B are diagrams illustrating a manufacturing process of a wavelength conversion element, in which FIG. 5A is a cross-sectional view illustrating a substrate, and FIG. 5B is a cross-sectional view illustrating a KN film and a KTN film stacked on the substrate. (C) shows the TiO film formed on the KN film. 2 It is sectional drawing which shows a film | membrane, (d) is sectional drawing which shows the wavelength conversion element after forming the confinement layer, (e) is sectional drawing which shows the wavelength conversion element after forming an electrode.
FIG. 6 is a drawing-substituting photograph showing the surface of the KN film, (a) is a drawing-substituting photograph showing the surface of the KN film formed directly on the MgO substrate, and (b) is a photograph formed by sandwiching the KTN film. It is a drawing substitute photograph which shows the surface of the formed KN film | membrane.
FIG. 7 is a perspective view showing a wavelength conversion operation in the wavelength conversion element.
[Explanation of symbols]
1 ... Board
2 ... KTN film
3 ... KN film
4 ... TiO 2 film
5 ... Confinement layer
6, 7 ... Electrodes
8 ... Diffraction grating
9 ... Power supply
S: Wavelength conversion element

Claims (8)

MgＯよりなる基板と、
光が伝播するＫNbＯ3膜と、
前記基板と前記ＫNbＯ3膜との間に形成されたＫTaXNb(1-X)Ｏ3膜と、
を備え、
前記ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面で４０原子％以上６０原子％以下であると共に前記ＫNbＯ3膜に接する面で０原子％となり、且つ、前記ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面から前記ＫNbＯ3膜に接する面に向かう方向に減少していることを特徴とする波長変換素子。A substrate made of MgO;
A KNbO 3 film through which light propagates;
A KTa x Nb (1-x) O 3 film formed between the substrate and the KNbO 3 film;
With
The value of the KTa X Nb (1-X) O 3 mixed ratio of Ta in the film X is the KTa X Nb (1-X) O 3 film wherein a plane in contact with the substrate 40 atomic% to 60 atomic% in the following And 0 atomic% on the surface in contact with the KNbO 3 film, and decreases in a direction from the surface in contact with the substrate of the KTa X Nb (1-X) O 3 film toward the surface in contact with the KNbO 3 film. A wavelength conversion element characterized by comprising: 請求項１に記載の波長変換素子において、
前記ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面と前記ＫNbＯ3膜に接する面との間で連続的に変化していることを特徴とする波長変換素子。The wavelength conversion element according to claim 1,
The value of the KTa X Nb (1-X) O 3 mixed ratio of Ta in the film X is the KTa X Nb (1-X) O 3 film wherein the the surface contacting the substrate the KNbO 3 in contact with the film surface of A wavelength conversion element characterized by continuously changing between. 請求項１又は２に記載の波長変換素子において、
前記ＫNbＯ3膜が、ＫNbＯ3結晶における（００１）面を主面としていることを特徴とする波長変換素子。In the wavelength conversion element according to claim 1 or 2,
The wavelength conversion element, wherein the KNbO 3 film has a (001) plane in a KNbO 3 crystal as a main surface. 請求項１から３のいずれか一項に記載の波長変換素子において、
前記ＫTaXNb(1-X)Ｏ3膜の厚さが３００Å以上５００Å以下であることを特徴とする波長変換素子。In the wavelength conversion element as described in any one of Claim 1 to 3,
A wavelength conversion element, wherein the thickness of the KTa X Nb (1-X) O 3 film is 300 to 500 mm. 薄膜を積層することによりに形成される波長変換素子の製造方法において、
MgＯよりなる基板上にＫTaXNb(1-X)Ｏ3膜を成長させる第１成長工程であって、成長された前記ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面で４０原子％以上６０原子％以下であると共に前記ＫNbＯ3膜に接する面で０原子％となり、且つ、前記ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面から前記ＫNbＯ3膜に接する面に向かう方向に減少するように当該ＫTaXNb(1-X)Ｏ3膜を成長させる第１成長工程と、
前記成長したＫTaXNb(1-X)Ｏ3膜の表面に、光が伝播するＫNbＯ3膜を更に成長させる第２成長工程と、
を備えることを特徴とする波長変換素子の製造方法。In the method of manufacturing a wavelength conversion element formed by laminating thin films,
In a first growth step of growing a KTa x Nb (1-x) O 3 film on a substrate made of MgO, the Ta mixing ratio X in the grown KTa x Nb (1-x) O 3 film The value of the KTa X Nb (1-X) O 3 film is 40 atomic% or more and 60 atomic% or less on the surface in contact with the substrate and 0 atomic% on the surface in contact with the KNbO 3 film, and the KTa First growth for growing the KTa X Nb (1-X) O 3 film to decrease in a direction from the surface in contact with the substrate of the X Nb (1-X) O 3 film toward the surface in contact with the KNbO 3 film Process,
A second growth step of further growing a KNbO 3 film through which light propagates on the surface of the grown KTa X Nb (1-X) O 3 film;
A method of manufacturing a wavelength conversion element comprising: 請求項５に記載の製造方法において、
前記第１成長工程において、前記ＫTaXNb(1-X)Ｏ3膜におけるTaの混合比Ｘの値が、当該ＫTaXNb(1-X)Ｏ3膜の前記基板に接する面と前記ＫNbＯ3膜に接する面との間で連続的に変化するように当該ＫTaXNb(1-X)Ｏ3膜を成長させることを特徴とする波長変換素子の製造方法。In the manufacturing method of Claim 5,
In the first growth step, the value of the mixing ratio X of the KTa X Nb (1-X) O 3 Ta in film, wherein the said KTa X Nb (1-X) O 3 wherein the surface in contact with the substrate film KNbO A method of manufacturing a wavelength conversion element, comprising growing the KTa X Nb (1-X) O 3 film so as to continuously change between the surfaces in contact with the three films. 請求項５又は６に記載の製造方法において、
前記第２成長工程において、前記ＫNbＯ3膜を、ＫNbＯ3結晶における（００１）面を主面として結晶成長させることを特徴とする波長変換素子の製造方法。In the manufacturing method according to claim 5 or 6,
In the second growth step, the KNbO 3 film is crystal-grown with the (001) plane of the KNbO 3 crystal as a main surface. 請求項５から７のいずれか一項に記載の製造方法において、前記第１成長工程において、前記ＫTaXNb(1-X)Ｏ3膜を厚さが３００Å以上５００Å以下となるように結晶成長させることを特徴とする波長変換素子の製造方法。8. The manufacturing method according to claim 5, wherein in the first growth step, the KTa X Nb (1-X) O 3 film is grown to have a thickness of 300 to 500 mm. A method for producing a wavelength conversion element, comprising:
JP09239898A 1998-03-20 1998-03-20 Wavelength conversion element and method for manufacturing wavelength conversion element Expired - Fee Related JP3681893B2 (en)

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